Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field. When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water or fat become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis. An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z-axis and that varies linearly in amplitude with position along one of the x, y, or z-axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength and, in turn, on the resonant frequency of the nuclear spins along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MRI signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spins in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and know reconstruction algorithms.
Sizing of RF coil arrays may include a trade-off between adequate coverage of a region of interest to be imaged and suitable signal-to-noise ratios. For example, RF coil arrays need to be large enough to receive MR signals from the anatomy region of interest. On the other hand, the RF coil arrays cannot be made too large, otherwise the signal to noise ratio (SNR) of the arrays will be degraded due to the poor fitting factor of large coil arrays and large size of coil elements. This trade-off is complicated by the large variability of patient sizes. A given RF coil array may be too small to provide sufficient coverage to large size patients, yet the same coil array may be too big to fit the small size patients and result in low SNR or poor image quality. Thus, one fixed size of coil arrays may not fit all patients. However, the cost and complexity associated with multiple different sized RF coils arrays may preclude the use of different sized coils, thus degrading imaging for at least some patients.